On Fri, 2011-03-25 at 16:01 +0000, Steve Harris wrote:
> On 2011-03-25, at 15:41, Pat Hayes wrote:
> >
> > On Mar 25, 2011, at 10:05 AM, Sandro Hawke wrote:
> >
> >> Thanks for the detailed answer, but I'm pretty sure you're answering a
> >> different question than I meant. (Sorry for not being more clear.)
> >> What I meant was: is OWL 2 Full okay with people Skolemizing ontologies
> >> they are asserting?
> >>
> >> I might be misunderstanding, but it seems like all the problems you
> >> point out only arise during the entailment check. And yes, I know you
> >> can't Skolemize a query. I would never even think about doing that.
> >> I'm just talking about Skolemizing assertions.
> >>
> >> I think its general best to do queries in a query language and/or a rule
> >> language, but maybe that's a matter of taste.
> >>
> >> You say, "you never know how someone will use your graph", so I guess
> >> the point is that Alice might publish an ontology that gets Skolemized
> >> by her system, and then Bob publishes an identical ontology, and then
> >> when Charlie comes along and wants to find out whether Bob and Alice's
> >> ontologies entail each other, he's going to get a false negative because
> >> of the Skolemization.
> >
> > We can probably even fix this, in fact. If we can reliably distinguish 'bnode URIs' from other URis, eg if they all use a common namespace, then there is an obvious notion of graph equivalence which allows a 1:1 replacement of the skolem URIs. And then Charlie can discover that, though not logically equivalent, Alice and Bob's graphs are graph-equivalent. People will write code to check things like this if it ever starts to matter to anyone. The cost of testing this is identical to the cost of checking graph equivalence right now (its the same algorithm.)
>
> Exactly.
>
> In triplestores I'm familiar (admittedly not that many) with bNodes are skolemised into a value space that's different from both literals and URIs, so this is a natural consequence.
So, is there a simple way we can flag them? I know it's out of scope
for the RDF WG to define one, but maybe there's a solution that's so
simple everyone can just start doing it without a W3C process.
Strawman 1: make new URI scheme for this
Con: very hard to do. (It took me and Tim Kingberg 4+ years to get the
"tag:" URI scheme RFC published. Hopefully it's gotten much easier, but
still I'm hesitant.)
Con: it wouldn't be a link for linked data
Strawman 2: use urn:uuid:<uuid>
Con: there might be some false-positives, because of people using UUIDs
who don't mean them like this
Con: might be longer than necessary
Con: no helpful human-readable element
Con: no link for linked data
Strawman 3: use tag:w3.org,2000:Skolem:<some optional
text>:<uuid>
Con: no link for linked data
Con: might be longer than necessary
Strawman 4: use any IRI with some magic string in it, like
"SkBNode" or "$+SKNB+$".
Con: some false positives, as magic string may appear in a few IRIs
where it was not intended (such as blog posts about the concept, which
use it in the title, or other naive machine generated URLs).
For me, the clear winner is Strawman 4, because I really like being able
to dereference stuff, even if it's a Skolem constant. This allows the
Skolemizer to provide web service if it wants to. You can also use 4
with a tag: URI if you don't want to support dereference.
-- Sandro